The present invention relates to a vacuum-operated brake booster of the type having a booster piston that is sealed in relation to the vacuum booster housing by a rolling diaphragm, and a tappet coupled to a brake pedal for the actuation of a double acting valve. By means of the double acting valve, a working chamber of the brake booster is alternatively connectible to vacuum or higher differential pressure. A first valve portion is formed by a valve seat at the booster piston and a sealing surface of sealing ring preloaded in the direction of the valve seat, while a second valve portion is formed by the same sealing ring and a valve piston connected with the tappet.
A vacuum booster of this general type is known (German printed and published patent application No. 29 42 416). In that patent, a booster piston subdivides the interior of a booster housing into a first working chamber, that is inwardly connected to a vacuum source, and into a second working chamber. A valve cylinder projects from the back side of the booster piston and contains, at its front side, a first annular valve seat with a valve piston being slidably arranged in the valve cylinder. The valve piston carries a second annular valve seat which is encompassed by the first valve seat and is coupled to a driving rod. The valve cylinder also receives a valve member which is urged by preloading against the first and the second valve seat. The first and the second working chamber establish communication through a gap between the first valve seat and the valve member upon retraction of the valve piston. On advance movement of the valve piston, the second working chamber is connected with the atmosphere via a space between the second valve seat and the valve member. Between the booster housing and the driving rod in this vacuum booster is a stop which confines the travel of retraction of the driving rod such that the distance between the first valve seat and the valve member is shorter in the inactive position of the driving rod than this distance is when the driving rod is to be retracted.
This known vacuum-operated brake booster has the disadvantage that, although the lost travel can be reduced until response of the device, it cannot be eliminated (for design reasons) since the tolerances of the individual components constituting the control, influence the lost travel with a view to preventing leakage of the device. Likewise, in other comparable known brake boosters, special arrangements make the valve piston approach the control housing sealing seat by about the "response lost travel".